Retroreflective articles have the ability to redirect incident light back towards the light source. This unique ability has led to the wide-spread use of retroreflective articles on various substrates. For example, retroreflective articles can be used on flat inflexible substrates, such as road signs and barricades; on irregular surfaces, such as corrugated metal truck trailers, license plates, and traffic barriers; and on flexible substrates, such as road worker safety vests, a jogger's shoes, roll up signs, and canvas-sided trucks.
There are two major types of retroreflective articles: beaded articles and cube-corner articles. Beaded articles commonly use a multitude of glass or ceramic microspheres to retroreflect incident light. Typically, the microspheres are partially embedded in a support film, and a specular reflecting material is provided between the layer of microspheres and the support film. The reflecting material can be a metal layer (for example, an aluminum coating as disclosed in U.S. Pat. Nos. 3,700,478 and 4,648,932) or an inorganic dielectric mirror made up of multiple layers of inorganic materials that have different refractive indices (as disclosed in U.S. Pat. Nos. 3,700,305 and 4,763,985). Categories of beaded articles include exposed lens, enclosed lens, and encapsulated lens types. Exposed lens beaded articles have a layer of microspheres that are exposed to the environment. Enclosed lens beaded articles have a protective layer such as a transparent polymer resin contacting and surrounding the front side of the microspheres. Encapsulated lens articles have an air gap surrounding the front side of the microspheres and have a transparent film hermetically sealed to a support film to protect the microspheres from water, dirt, or other environmental elements.
In lieu of microspheres, cube-corner sheeting typically employs a multitude of cube-corner elements to retroreflect incident light. The cube-corner elements project from the back surface of a body layer. In this configuration, incident light enters the sheet at a front surface, passes through the body layer to be internally reflected by the faces of the cube-corner elements, and subsequently exits the front surface to be returned towards the light source. Reflection at the cube-corner faces can occur by total internal reflection when the cube-corner elements are encased in a lower refractive index media (e.g., air) or by reflection off a specular reflective coating such as a vapor deposited aluminum film. Illustrative examples of cube-corner sheeting are disclosed in U.S. Pat. Nos. 3,712,706; 4,025,159; 4,202,600; 4,243,618; 4,349,598; 4,576,850; 4,588,258; 4,775,219; and 4,895,428.